Dear EarthTalk: What, if anything, fills the empty space underground created by the extraction of billions of gallons of oil? Could oil drilling be one of the causes of increasing amounts of land settling and sinkholes in oil rich areas? Can it cause earthquakes?

—Linda Anderson, Sedona, AZ

The crude oil (and natural gas) we drill for the world over is, for the most part, stored in tiny pores within rock up to only about three miles deep in the Earth’s hugely dense crust. At such depths, the oil there is under fairly high pressure. When it is removed, other liquids—usually water—move in to take its place, equalizing the pressure in the process. Sometimes oil extractors pump water into one side of an oil field to push oil toward wells on the other side, and the water replaces the oil accordingly.

In cases where other liquids don’t move in, such as in the North Sea off The Netherlands, the porous rock layer that harbored the oil originally can collapse after extraction, causing slight amounts of land settling (known as "land subsidence") in the rock layer surfaces above, but typically no more than a few tenths of an inch per year.

Here in the U.S., land subsidence induced by the large volume extraction of underground resources including oil and gas "is more common than most people realize," according to the U.S. Geological Survey (USGS), a government agency which collects, monitors, analyzes and provides scientific understanding about natural resource conditions, issues and problems. Flat coastal plains and wetlands near sea level are most at risk from this potential side effect.

Excessive ground water pumping, not oil or gas extraction, is the single largest source of land subsidence, says the USGS, but the agency cites several cases throughout the 20th century which they say demonstrate how "accelerated withdrawal of oil, gas and associated water from shallow unconsolidated reservoirs could lower the land elevation, cause minor earthquakes, and activate faults [around oil fields]."

Subsidence around large, mature oil and gas fields that coincide with faults could add enough stress to trigger small, locally based earthquakes as far as two kilometers away from the offending wells. Most geologists agree, though, that it is unlikely that oil and gas extraction could contribute to or cause major earthquakes, which are generated at depths far deeper than would be practical to drill for oil or gas. The USGS does suggest, however, that the continued withdrawal of oil and gas and the associated decline in underground fluid pressure could even contribute to coastal sea level rises by lowering coastal land elevations.

As for sinkholes, modern oil wells tend to be much deeper than the depth where sinkholes typically can affect people. Nonetheless, in 1980 residents of the West Texas town of Wink awoke one morning to find a 370-foot wide, 110-foot deep sinkhole a couple of miles north of downtown. Geologists suspect the sinkhole formed as a result of historic (and by today’s standards outdated) oil production practices in the area whereby extractors pumped saltwater out from underneath the surface and left a void that the above layer of earth eventually collapsed into. A second, even bigger sinkhole opened up nearby in 2002.

In the U.S., teeth whitening products are not regulated by the U.S. Food and Drug Administration, as they are not classified as drugs. As such, long term safety data doesn’t exist for them. But health experts warn that consumers should beware of the risks of using stronger varieties containing hydrogen peroxide. Hydrogen peroxide tends to be more effective (it essentially bleaches the tooth enamel), but it is a harsh chemical that can be poisonous if swallowed.

Europa, the official website of the European Union (EU), cites studies showing that bleaching teeth with hydrogen peroxide-based products can "harm the surface of the teeth, making the enamel more porous and leading to dents, scratches and loss of minerals." Europa further warns that it’s important for people to keep their tooth enamel in good condition as it is "the protective, hard layer covering the softer dentine inside the tooth" and "does not regenerate." The EU recommends people avoid tooth whitening products with hydrogen peroxide levels higher than a 1.5 percent concentration; most over-the-counter varieties come in at about a 0.5 percent concentration level. If the label on the product you are considering doesn’t indicate the concentration, it might be better to go with one that has a more complete ingredients listing.

Dentists can access teeth whitening solutions with higher concentrations of hydrogen peroxide than are available over-the-counter; as such a professional job in your dentist’s office will be more effective and last longer than the solutions you can take home from the drug store. And while higher concentrations of hydrogen peroxide might not be what you’re looking for, dentists can apply it in more targeted ways. If you do it yourself at home there is a greater chance you will expose your gums and other parts of your mouth to hydrogen peroxide or swallow more of it than you should.

As for maintaining that bright white look, whether you did it yourself or had it done professionally, your local drugstore or supermarket no doubt carries a wide selection of toothpastes that claim to whiten teeth. The ones which work the best contain—you guessed it!—hydrogen peroxide, which can be irritating if used day after day.

Fortunately for the health-minded home teeth whitener there are many less harsh varieties of these toothpastes now on the market. The website Skin Deep, a free online safety guide to cosmetics and personal care products published by the non-profit Environmental Working Group, lists Tom’s of Maine Natural Antiplaque Tartar Control Plus Whitening Toothpaste—which makes use of all-natural hydrated silica, not hydrogen peroxide, for whitening and stain removal—as one of the safest kinds of whitening toothpastes out there today. Burt’s Bees Natural Fluoride-Free Whitening Toothpaste and CloSYS Toothpaste for Teeth Whitening also get high marks from Skin Deep for their natural, non-toxic ingredients. While such products may not be "advanced" formulations from a leading packaged goods conglomerate, your teeth and body may thank you later.

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